MARS’S MISSING MELT SHEETS: ARE THEY HIDDEN WITHIN OUR SHERGOTTITE COLLECTION?

Impact cratering is a major process that has shaped planetary surfaces. Impacts generate large-scale morphological structures as well as small-scale uniquely diagnostic micro-scale mineral transformations. Impacts also generate new rocks through the process impact melting which can, in some cases, lead to slowly cooled impact-generated igneous rocks. On Mars, interpretations of igneous rocks as impact products are uncommon, despite the heavily cratered surface and the preference for remote missions to target craters for exploration. Similarly, martian meteorites, particularly the most abundant type, shergottites, are typically interpreted as volcanic, emplaced as shallow intrusions or thick lava flows.

Importantly, impact melt rocks crystallize after the passage of the shock wave, and therefore these rocks lack the shock metamorphic features used to uniquely identify shocked rocks. In this case, impact melt rocks have many of the same characteristics of traditional endogenic melts and distinguishing them may be difficult from remote observations or in samples which lack context such as meteorites. Here, we present ²⁰⁶Pb/²⁰⁴Pb isotope ratios in multiple single feldspar grains from individual hand samples of impact melt rock from the Manicouagan and Sudbury impact structures. Our results show an extreme range of ²⁰⁶Pb/²⁰⁴Pb values indicating that the melt sheet is not homogeneous with respect to Pb at the mm scale. This level of heterogeneity is significantly larger than what is seen in non-impact-generated igneous rocks. We conclude that different formation mechanisms for impact- versus non-impact-generated melts lead to distinct Pb isotopic behavior that is recorded within feldspars at the time of crystallization. Individual Pb isotope ratios of feldspars from martian shergottites show a large range in ²⁰⁶Pb/²⁰⁴Pb values within one sample, more similar to the terrestrial impact melt sheets than to non-impact igneous rocks. We suggest crystallization from an impact melt sheet rather than volcanic source as a petrogenetic model for some of the martian shergottites.